Higher-order skin effect and its observation in an acoustic kagome lattice

The non-Hermitian skin effect (NHSE) is a distinctive topological phenomenon observed in non-Hermitian systems. Recently, there has been considerable interest in exploring higher-order NHSE occurrences in two and three dimensions. In such systems, topological edge states collapse into a corner while bulk states remain delocalized. Through a Hermitian-non-Hermitian correspondence, this study predicts and experimentally observes the higher-order NHSE in an acoustic kagome lattice possessing nonreciprocal hoppings. By rotating the frequency spectrum and employing complex-frequency excitation techniques, we observe the localization of acoustic energy towards a corner of the lattice in the topologically nontrivial phase, even when the source is located far from that corner. In contrast, the acoustic energy spreads out when excited at the frequencies hosting the bulk states. These observations are unequivocal evidence of the higher-order NHSE.